The invention relates to a power-driven electrical hand tool according to the preamble of claim 1.
Electrical hand tools that have a coupling between the driving electric motor and the output shaft of the tool are known in many forms, primarily in "electric screwdrivers," i.e. screwdriving tools that are intended to be made capable by means of at least one such coupling, when a predefined torque is reached, of interrupting power transfer, preferably noiselessly, i.e. without any reciprocal sliding or chattering of the coupling claws of the said coupling as they release from engagement.
Particular discussion will also be devoted below to publications in the screwdriver area relating to the prior art, although it is understood and hereby noted that the invention is not confined to electric screwdrivers or screwdriving tools, but can be utilized, in the context of power-driven electrical hand tools, wherever abolition of the drive connection between driving electric motor and tool output shaft is desired.
For example, in a motor-driven automatic-shutoff screwdriver (DE-OS 30 15 423), it is known that when a preset limit torque is exceeded, a snap-lock coupling makes contact and thereby moves a coupling element by the fact that cylindrical rollers held in depressions exert pressure on a coupling sleeve, thus causing a locking apparatus to respond.
The latter then firmly holds the said coupling element in its position attained when the snap-lock coupling made contact, and the entrainment coupling is not positively engaged again until the tool spindle has been pushed in past a position identified by a pressure point. Noiseless coupling cannot, however, be achieved with such a coupling; instead the various operating ranges, as mentioned explicitly in said DE-OS 30 15 423, are clearly distinguishable by the difference between continuous rotation in the normal range and pulsed rotation in the overload range.
However, precisely in order to prevent the claws of the coupling from continuing to strike against one another if loading persists (the screwdriver still being engaged in the screw head) once a limit torque, adjustable by means of a depth stop, has been reached--which can cause considerable noise and substantial wear on the claws--an attempt has been made elsewhere (DE-OS 35 10 605) completely to disengage the positive working connection to the drive-side coupling element. For this purpose, there is arranged between the two halves of the claw coupling provided a spring-loaded intermediate disk, also having claws or cam projections on both sides, which sits on the tool drive shaft in a freely rotatable and axially displaceable manner. At its end surfaces, this intermediate disk is linked on one side by means of cams and on the other side by means of claws to the correspondingly and complementarily configured adjacent coupling elements, the claws, on one side, having axially elevated oblique surfaces so that the intermediate disk can also be displaced axially. At the moment at which the limit torque is reached, or at which the axially displaceable drive shaft has moved so far outward (relative to the depth stop) that the cams of one of the coupling regions can for the first time no longer transfer torque, the intermediate disk allows complete, spring-aided separation of these coupling elements, since at this moment on the other side of the intermediate disk the claws slip off the oblique surfaces, thereby additionally opening up the axial clearance in the cam transfer region of the coupling. The additional placement of the intermediate disk and the corresponding increase in space requirement can present problems here, making the overall structure more complex. In particular, this intermediate disk--free to move axially and radially--constitutes an additional member of the unit that is subject to wear and may therefore possibly increase its susceptibility to malfunction.
A further known coupling for power-driven screwdriving tools (DE-PS 36 37 852) attempts to address this problem by completely omitting the additional intermediate disk and configuring the transition region between the coupling element of the claw coupling on the drive shaft side and the drive shaft itself in a particular manner, forming a drag coupling. This drag coupling allows relative rotary motion between the drive shaft and the said coupling element over only a small angle, and is implemented by the fact that a transverse pin arranged nonrotatably on the drive shaft engages in blind holes or oblique transverse guides on the coupling element. A concurrent result of these oblique stop surfaces is that as the transverse pin shifts in its guides, an axial relative displacement occurs between the coupling element and the drive shaft, ensuring, in a known manner, that as the unit uncouples when the desired screwdriving depth is reached, the claw coupling separates completely as soon as the transferred torque disappears (i.e. when the limit torque is reached) by additionally increasing the clearance.
Lastly, this kind of solution according to DE-PS 36 37 852 is amplified upon by a further known power-driven screwdriving tool (EP 0 382 149 A1) in which, while the fast uncoupling processes effected by the oblique surfaces in the transition region on the drive-shaft-side coupling element are retained, at least one open recess, oriented axially outward from the screwdriving tool, is configured in the form of a guide pocket, the base of the recess running obliquely with respect to the drive shaft's long axis. A transverse pin nonrotatably mounted on the drive shaft engages in this guide pocket to form the drag coupling.
The underlying object of the invention is to improve these power-driven electrical hand tools--some of which are extremely complex in terms of both configuration and function--so that a considerably simpler structure can be attained while retaining smooth, noiseless uncoupling when the predefined screwdriving depth is reached.